bush, bill clinton and jimmy carter. the measure of mandela so great, america's presidents shared a stage with cuban leader, castro. >> for peace without, for peace within. >> reporter: after interfaith prayers, the tributes began. first, andrew melangani, whose cell on robert island was next to mandela's. >> my heart, my soul, my life. >> reporter: then members of mandela's family spoke, including four of his grandchildren. >> it's all over the world like a comet. >> reporter: president obama, who received a warm welcome, told the crowd how mandela's example helped guide his own path. >> but i believe it should also prompt in each of us a time for self reflection. with honesty, regardless of our station or our circumstance, we must ask, how well have i applied his lessons in my own life. >> reporter: the crowd, overcome with emotion. mandela's life held up as a call to action, that the work to end oppression is far from over. security on heightened alert for this impressive gathering of world leaders, many seated behind